The invention relates to an electric motor and more particularly relates to a 3-phase Hall motor having two Hall generators used in combination with a control circuit including comparator circuits, a logic circuit and a three-phase inverter circuit which are sequentially operated in response to an output from the Hall generators to control the rotation of the motor. Moreover the electric motor has a pair of armatures and a pair of cylindrical rotors each of which is rotated around each of the armatures, the cylindrical rotors being pressed against each other at one end thereof and having the one end secured to a rotating shaft to maintain the mechanical balance of the rotors and to increase the balance of magnetic force acting between the armatures and the rotors to thereby rotate the motor at a high speed with reduction of vibrations. Further the armatures have a predetermined number of electrodes, the electrodes of one armature being arranged as angularly displaced from those of the other armature to smooth the rotation of the motor.
The Hall motor is generally simple in mechanism, free from mechanical and electric noises, unabrasive except for the bearings, and is therefore durable, reliable, requires no maintainance work and is adapted to rotate at a high speed. With so many features being provided, the Hall motor of the invention may be widely used in the appliances such as audio devices, disk memory drives, facsimile scanners. etc.
So far Hall motors have been generally expensive because this type of motor requires extremely complicated control circuits including Hall generators which are of a high cost. Recently the Hall generators have been supplied in a mass production with the progress of semiconductor processing techniques and may be easily available for use in motors, but none the less the Hall generators are more expensive than the linear IC.
For example, in the conventional Hall motor of 3-phase and bipolar type which corresponds in energy conversion efficiency to the 3-phase motor having brushes, the rotor has a driving permanent magnet and a position detecting magnet secured thereto with the polarities N and S being separately provided, and the three Hall generators are arranged with an angular space 120.degree. being provided therebetween for detecting only one direction of magnetic flux of polarity N or S so as to produce each positional signal to thereby operate transistors for supplying the electric current to the coils of motor through a number of gate circuits. Such a Hall motor is costly because the magnetization of rotor is complicated, three Hall generators are required and especially because specific integrated circuits are required to provide so many gate circuits.
With the foregoing reasons, many Hall motors have been of 2-phase type with two Hall generators together with comparatively simple control circuits being provided. Such a 2-phase Hall motor however cannot avoid dead points in principle and often stops if the motor has a frictional load. Moreover when the torque is small, the counter electromotive force is small and the copper loss is considerably large and accordingly the efficience is extremely low. Further the starting torque is small. Therefore the 2-phase Hall motor has been used especially in combination with appliances such as fans which will not require a big starting torque, instead of appliances such as toys, cassette tape recorders, etc., which require a big starting torque. The brush type motors have been used for these appliances requiring a big starting torque. However it is generally known that the brush type motors produce electric noise, and the brushes and commutators are easily abrasive and accordingly are of poor durability.
Further the conventional Hall motors have an armature secured to the motor case and a cylindrical rotor secured to the rotating shaft and rotatable around the armature. More precisely the cylindrical rotor has one end secured to the rotating shaft. It is therefore very difficult to obtain the mechanical balance of the rotor and the balance of magnetic force acting between the armature and the permanent magnets of the rotor. Accordingly it is almost impossible to drive such a motor at a high speed, for example, at the speed of about 10,000 r.p.m. Further the considerable mechanical noise and vibration are unavoidable. These defects become more and more remarkable in case the armature is thickened in the axial direction of the rotating shaft, and the cylindrical rotor is accordingly lengthened in the same direction in order to increase the output of the motor.